Caterpillar SPN 658 FMI 5: Diagnosing an Open Cylinder #8 Injector Circuit
SPN 658 FMI 5 (FMI 5) — Engine Injector Cylinder #08 - Current Below Normal Or Open Circuit
Quick Answer
SPN 658 FMI 5 on Caterpillar C15/C16 engines means cylinder #8's injector circuit is reading current below normal, consistent with an open circuit. It's a high-severity fault causing a real misfire on that cylinder, most often traced to a harness connector issue rather than a failed injector.
Overview: Caterpillar SPN 658 FMI 5
A dead or open injector circuit on any single cylinder produces an unmistakable rough-running complaint, and SPN 658 pinpoints it directly to cylinder #8 on Caterpillar's numbering. ProfessionalDieselRepair.com works this fault methodically - pin-testing the harness connector at the valve cover before ever pulling the injector itself, since a corroded or backed-out pin produces the exact same open-circuit reading as a genuinely failed injector solenoid, at a fraction of the repair cost.
Repair Snapshot
Time
1.5 - 4 Hours
Difficulty
Advanced
Parts Cost
$150 - $900
Dealer Labor
$600+
Tools Required
- Cat ET (Electronic Technician)
- Diesel Laptops Scan Tool
- Jaltest Diagnostic Software
- Injector Torque Wrench
What SPN 658 FMI 5 Means
SPN 658 FMI 5 means the ECM commanded cylinder #8's injector solenoid or needle control valve but measured current below normal, consistent with an open circuit. On a Caterpillar C15/C16, this is a high-severity fault - that cylinder is likely contributing little or no power, causing a noticeable misfire and rough running until it's repaired.
Symptoms Drivers Notice
- Rough idle with a distinct miss you can feel and hear
- Reduced power and possible black or white smoke depending on the failure mode
- Check Engine lamp illuminated
- Increased fuel consumption as the ECM compensates on remaining cylinders
Most Likely Root Causes
Corroded or damaged injector harness connector
CommonThe connector at the valve cover is exposed to heat, oil, and vibration, making it a frequent point of failure that mimics an internal injector fault.
Broken wire within the injector harness
OccasionalRepeated flexing where the harness routes near the valve cover grommet can eventually fracture a wire internally, producing an open circuit.
Failed injector solenoid winding
OccasionalThe injector's internal solenoid coil can fail open after sufficient heat cycling and electrical stress over its service life.
ECM driver circuit fault for that specific cylinder
RareAn internal ECM issue affecting just one injector driver circuit is uncommon but should be considered if the harness and injector both test normal.
Common Misdiagnoses (Don't Waste Parts)
Replacing the injector immediately based on the fault code alone
Why it happens: The fault names the injector circuit directly, making the injector itself the obvious - and expensive - parts-cannon target.
Do this instead: Pin-test the harness connector and check resistance with the injector both connected and disconnected to isolate whether the fault is in the harness or the injector itself.
Assuming a rough idle on one cylinder is always a fuel quality issue
Why it happens: Rough running can have many causes, and fuel quality is an easy first guess.
Do this instead: Pull codes first - an active injector circuit fault points directly to the affected cylinder without needing to guess at fuel-related causes.
Master Tech Commentary
Mechanic's Notes
Shop tip: don't skip straight to injector replacement - pin-test the harness connector at the valve cover first. ProfessionalDieselRepair.com finds a surprising number of these 'dead cylinder' codes are actually a harness pin that backed out or corroded rather than a genuinely failed injector solenoid.
Step-by-Step Diagnostic Procedure
- Step 1: Pull the fault with Cat ET and confirm SPN 658 FMI 5 is active; check for any other injector-related codes that might indicate a broader harness issue rather than a single-cylinder problem.
- Step 2: Locate the injector wiring harness connector for cylinder #8 at the valve cover and inspect it closely for corrosion, a bent pin, or a connector that isn't fully latched.
- Step 3: Disconnect the connector and use a multimeter to check resistance across the injector solenoid winding, comparing the reading to the manufacturer's specification and to a known-good cylinder on the same engine.
- Step 4: If resistance reads infinite (open), trace the harness back toward the ECM looking for a break, often at a point where the harness flexes near the valve cover grommet.
- Step 5: If the injector itself tests open internally, replace that injector following the proper torque and calibration procedure required after Cat injector service.
- Step 6: Clear the code and run the engine, using Cat ET's cylinder cutout test to confirm cylinder #8 is now contributing power evenly with the others before returning it to service.
Professional service / OEM-level scan tools are recommended before clearing this fault for good.
Live Data & Freeze-Frame Tips
Use Cat ET's individual cylinder cutout test to confirm cylinder #8 is contributing little or no power compared to the others, which corroborates the fault code with an actual performance measurement. Compare injector resistance readings across all cylinders side by side to make an open circuit on #8 obvious relative to healthy cylinders.
Safety Warnings
- Disconnect the battery before disconnecting any injector harness connector.
- Allow the engine to cool before working around the valve cover and injector connectors.
- Follow proper torque specifications when reinstalling any injector to avoid fuel system leaks.
When to Limp In vs Tow
Towing generally isn't necessary for a single dead cylinder - the engine will run, just roughly and with reduced power. If power loss is severe enough to be unsafe in traffic, arrange service before continuing.
Prevention Tips for Fleets & Owner-Operators
- Inspect injector harness connectors for corrosion during valve cover service or adjustment intervals.
- Keep the valve cover gasket in good condition to prevent oil intrusion into the connector area.
- Address any minor injector circuit faults promptly before they progress to a full open circuit.
Related Fault Codes
SPN 658 FMI 6
Same cylinder #8 injector circuit flagged as current above normal (shorted) instead of open - a different electrical failure mode.
SPN 657 FMI 5
Cylinder #7 injector circuit open - the adjacent cylinder's equivalent fault, useful to check if harness damage is spreading across multiple cylinders.
SPN 651 FMI 5
Cylinder #1 injector circuit open - the same fault type on a different cylinder, worth checking if a broader harness issue is suspected.
Frequently Asked Questions
Can I drive with SPN 658 FMI 5 active?
You can drive short distances, but expect a rough idle, reduced power, and poor fuel economy. Get it repaired soon rather than running the engine on fewer cylinders long-term.
How do I reset SPN 658 FMI 5 without a scan tool?
You need Cat ET or a comparable diagnostic tool to clear the stored fault after the harness or injector issue is repaired.
Does this code always mean I need a new injector?
No - check the harness connector for corrosion or damage first. A significant number of these faults are wiring issues rather than a genuinely failed injector.
Will this code cause an engine derate?
A single dead cylinder can reduce overall power output, and depending on the engine's protection strategy, a sustained multi-cylinder misfire condition could trigger additional protective measures.
How can I tell if it's the injector or the harness without full diagnostics?
Check injector resistance with the connector disconnected directly at the injector versus at the harness connector - if resistance is normal at the injector but reads open at the harness end, the wiring is at fault.
Is a dead cylinder from an open circuit reversible without permanent damage?
In most cases yes, as long as it's addressed reasonably promptly - extended operation with unburned fuel entering that cylinder can eventually contaminate oil or affect the exhaust system, so don't let it go too long.